The present invention relates to a radio frequency (RF) electrical power generator that includes an induction coil for exciting inductively coupled plasma (ICP) for use in spectrometry.
In spectrometry, an ICP may be used as a source of ions for mass spectrometry (MS) or as a source of light for atomic optical emission spectrometry (OES). The ICP apparatus typically consists of one or more water-cooled induction coils mounted coaxially with a torch assembly through which argon gas is introduced at carefully controlled rates. RF electrical power is supplied to the induction coils. The argon does not conduct electricity and so is not heated by the RF electromagnetic field of the induction coils until the plasma is “struck”, typically by a small electrical discharge created in the argon by a high voltage spark. This discharge produces enough electrons and ions to interact with the RF electromagnetic field and thereby generate sufficient heat for the process of ion and electron generation to become self-sustaining, thereby sustaining the plasma.
In spectrochemical applications the temperature of the plasma is typically in a range of 3,000 K to 10,000 K. The frequency of the varying electromagnetic field can be in the range of a few MHz to many GHz, but a particularly useful range, where it is relatively easy to excite the plasma to an appropriate temperature is between 10 MHz and 50 MHz.
The generator for supplying the RF electrical power must be capable of generating sufficient power for exciting and maintaining the plasma, typically in the region of 500 W to 3 kW. It must also be capable of handling rapid and significant changes in the load impedance, as occurs for example when plasma unexpectedly extinguishes. It must also be able to handle non-constant load impedance conditions as occurs for example between excitation and sustained generation of the plasma.
Another significant factor is the cost of RF generator in a spectrometry instrument. Spectrometry instrumentation is expensive and the field is competitive, thus there are cost constraints on componentry such as the RF generator or oscillator if a manufacturer of such instruments is to remain competitive in the market.
An object of the present invention is to provide a relatively inexpensive RF electrical power generator for exciting and sustaining inductively coupled plasma for spectrometry. This object is in part met by the provision of a RF oscillator that comprises solid state components.